The fluffernutter is a peanut butter and jelly sandwich with the jelly swapped for marshmallow creme, and that one substitution changes what kind of sandwich it is. Peanut butter goes on one slice of soft white bread, a thick layer of marshmallow creme goes on the other, and they are pressed together. The peanut butter is the structural element, not the headline flavor: spread to the bread, it waterproofs the crumb against the wet, sugary creme so the sandwich holds rather than dissolving into a sticky smear. The Fluff is what pushes it fully into dessert. This is the whole sandwich, two spreads and a knife, and the engineering is in how those two spreads behave against each other.
As a sandwich it works on ratio and on bread choice. Marshmallow creme is looser and far sweeter than jelly, so the peanut butter has to go on heavier to hold the structure and to keep the sweetness from becoming the only thing the mouth registers. The bread is deliberately soft and faintly sweet white sandwich bread, because a crust with real chew fights a filling that has no texture of its own and a denser loaf would overwhelm a center that is almost entirely soft. The two layers are spread to separate slices and joined at the end rather than stacked on one, so neither soaks the bread before the sandwich is closed. The result is intentionally one-note in texture, soft against soft, and it is held together by the peanut butter doing the same waterproofing job it does in a standard PB&J, just against creme instead of fruit.
The fluffernutter sits in the American peanut butter and jelly family, the sweet-sandwich shelf whose founding rule is a soft carrier, a spreadable anchor, and a sweet center that has to stay put. Its relations keep that frame and change the sweet half or the assembly: the banana version that adds sliced fruit, the grilled fluffernutter that turns the spreads molten in a buttered pan, and the standard jelly PB&J it descends from. Those deserve their own articles rather than being crowded in here.